d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of fiber aspect ratio and volume loading on the flexural properties of flowable dental composite Paul Shouha, Michael Swain, Ayman Ellakwa ∗ Biomaterials Science, Discipline of Biomaterials, Faculty of Dentistry, The University of Sydney, Sydney Dental Hospital, 2 Chalmers St., Surry Hills, Sydney, NSW 2010, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To evaluate the efficacy on flexural properties of flowable dental resin composite
Received 25 September 2013
reinforced with short glass fiber of different aspect ratios (ARs) and volume percent load-
Received in revised form
ings. It is hypothesized that with the addition of randomly oriented fibers it is possible to
2 April 2014
significantly improve flexural strength and modulus while maintaining flowability.
Accepted 8 August 2014
Methods. Ten groups of samples with varying glass fiber volume loads (0, 5%, 10%, 20%, 40% and 60%) and three different ARs (5.2, 68 and 640) were tested in three point bending to fracture according to ISO 4049. A flowable resin composite was used as the control and also
Keywords:
as the filled resin composite that was subsequently reinforced with fibers. Load deflection
Flexural
results were used to calculate flexural strength and flexural modulus. SEM images were
Fracture
used to determine the mode(s) of failure, to describe surface features of reinforcement and
Strength
were correlated with force displacement graphs. All results were statistically analysed using
Modulus
ANOVA followed by post hoc Tukey’s test. Level of significance was set at 0.05.
Dental composite
Results. When compared to the “sculptable” control (68.6 vol% filler loaded) results for flex-
Flowable
ural strength varied from a mean reduction of 42% (p > 0.05) for the low AR group to an
Glass fiber
increase of 77% (p < 0.001) for the high AR samples. Flexural modulus results varied from a
Aspect ratio
low of 6.6 [0.67] GPa for the non reinforced spatulated control to 20.3 [1.31] GPa (p < 0.001) for the 60% loaded low AR group. The low fiber loaded mid AR group was still flowable with 49% total loading (5% fiber/44% filler) but gave strength values (181.2 [33.5] MPa) 30% higher than the “sculptable” control (p > 0.05) and comparable modulus. Significance. This study shows that short and very short glass fibers can significantly reinforce flowable dental composite. The fiber’s aspect ratio was shown to be more important than volume loading for flexural strength. It appears possible to produce a light cured short glass fiber reinforced flowable material with superior flexural properties compared to conventional universal composites. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author: Associate professor, Faculty of Dentistry, The University of Sydney, Westmead Oral Health Centre, Level 1, Westmead Hospital, Darcy Road, Sydney, NSW 2145 Australia. Tel.: +61 298457161; fax: +61 296334759. E-mail address: [email protected] (A. Ellakwa) .
http://dx.doi.org/10.1016/j.dental.2014.08.363 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
1.
Introduction
Whether driven by public esthetic demands, concerns over amalgam’s toxicity or trends toward conservative management of dentitions, dental composites have become the material most often employed in the restoration of teeth. These materials are composed of varying amounts of inorganic radio opaque particulate fillers dispersed in a relatively weak organic resin matrix. Bi-functional silane compounds are used to create the interphase between the two. These materials are manipulated directly in the mouth and are set with curing lights of approximately 470 nm wavelength. Their use in stress bearing areas of posterior teeth is becoming an acceptable option but brittleness and a tendency to edge fracture remain important limitations [1]. There has been a long and successful history of high performance fiber reinforcement of industrial composites. This cannot be said of dental composites. Although relatively uncommon clinically, the introduction of fibers have been studied in fixed and removable dental prostheses [2,3]. These fiber systems though are usually continuous and are indirectly manufactured [4–6]. Compared with direct procedures these restoratives require more aggressive tooth preparation. Other limiting factors include significant laboratory fees and the increased probability of handling and processing errors. Chair side reinforcement with continuous fiber composite is still less common. This direct procedure involves combining regular dental composite with sections cut from straight or woven ultra high molecular weight polyethylene fibers. Not only has fiber reinforcement been shown to strengthen and toughen dental composite but direct reinforcing was also found to strengthen restored teeth [7]. Additionally, the fracture type changes from a catastrophic brittle pattern to a more graceful controlled one. Least common are dental composites reinforced with randomly oriented short fibers. These materials claim higher stiffness, fracture toughness and flexural strength. For example: 16 GPa, 3.2 MN/m3/2 and 176 MPa respectively for a 71% volume filled, 20–800 m long E glass rod material [8,9]. They also claim very low polymerization shrinkage [10] but have been found to exhibit poor wear properties due to “plucking” of the relatively large surface exposed fibers and fillers [11]. Variables such as fiber position and orientation have been studied [12–14]. Using chopped glass fibers in denture polymers Karacaer [15] found significant correlation between fiber volume with both flexural modulus and impact strength. Callaghan [16] found that increasing the length of the fibers increased the wear resistance of reinforced specimens. The intent of this study was to evaluate the effect of different fiber aspect ratios and fiber volume loads on the flexural strength and modulus of low viscosity or flowable dental composites. Fiber aspect ratio (AR) is simply the relationship of length to diameter of a fiber given as a number. Some studies have quantified AR as a variable for mechanical properties of dental materials [16–18] but overall there exists very little research in this field and none that we know of with regards to flowable dental materials. The hypothesis
1235
evaluated was that a flowable dental composite may be significantly reinforced by incorporating randomly distributed short and very short glass fibers. Further it was hypothesized that, the amount of fiber required to increase the flexural behavior is small thereby retaining low viscosity and flowability.
2.
Materials and methods
Ten groups (A–J) with ten samples per group were formed. Three control groups A, B and C were not reinforced while groups D–J formed the fiber reinforced samples (Table 1). Group A consisted of a universal high viscosity or “sculptable” dental composite (Beautifil II, Shofu, Kyoto, Japan) while groups B and C consisted of a low viscosity flowable composite (Beautifil F03 Flow Plus). Groups D–F were formed by combining this flowable material (F03 Flow Plus) with low (5.2) aspect ratio fibers of different volume load fractions (20, 40 and 60%). Groups G, H and I combined F03 Flow Plus with mid (68) aspect ratio fibers of three different loads (5, 10 and 20%) while group J utilized high aspect ratio (640) fibers with 10% loading.
2.1.
Fibers
Glass fibers with three different aspect ratios (ARs) were used (Fig. 1). Low AR fibers presented as unsized milled E glass powder from Owens Corning, Ohio, USA. SEM analysis revealed an average length of 89 ± 62 m (range 14–300, n = 400), an average diameter of 17 m and an AR of 5.2 ± 3.65. They were used for three groups (D–F). High AR glass fibers presented as S glass (S2 Glass) chopped strands from AGY, South Carolina, USA and consisted of numerous 10 m diameter parallel filaments cut into short 6.4 mm lengths. These fibers have an AR of 640 without deviation and were used for group J. These S glass strands were also cut (average length 679, range 60–2400, n = 400) to create the third AR of 68 ± 43.5 used in 3 groups (G–I). Both E glass and S glass fibers were used as the availability of very short cut or milled glass fibers is very limited. The prefix E and S stands for electrical grade and structural grade. Their physical properties are as follows: tensile strengths 3.4 GPa vs. 4.9 GPa, moduli 72 GPa vs. 87 GPa and densities of 2.58 g/cc vs. 2.46 g/cc respectively. Both fibers are predominantly aluminosilicate glasses. E glass is high in calcium while S glass is high in magnesium with no boron [19]. Given the similarity in composition, physical properties and surface chemistry it was decided to use milled E glass as a low AR baseline. This material is readily available and is used by manufacturers of “packable” DRCs. The S glass fibers were used uncut in the higher AR sample group. The major difference between the two glasses was thus considered to be diameter and a sufficient spread of aspect ratios was achieved. All fibers were first etched in a 4% potassium dichromate(IV)(K2 Cr2 O7 )/89% sulphuric acid (H2 SO4 ) aqueous solution (Australian Scientific, NSW, Australia) for 4 h then thoroughly washed (until pH returned to 7) in deionized water followed by rinsing in ethanol and drying in an oven at 50 ◦ C for 8 h. Fibers were then immersed in a hydrolyzed
1236
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
Table 1 – Ten groups (n = 10): three controls (A–C); three low aspect ratio (D–F); three mid AR (G–I) and one high AR (J) groups. Mean values and one standard deviation (brackets) are given. Note that only group G remained flowable after fiber incorporation. ef , effective stress yielding; Eef , effective modulus.
Group
Composion
Resin
Flowability
Flexural Strength mean
%
(SD)
MPa
σef
Flexural Modulus mean (SD)
GPa
Eef
A
Beautifil II universal giomer composite
31.4
“Sculptable”
139.8
(10.6)
-
12.2
(0.74)
-
B
Beautifil Flow Plus F03 flowable giom er syringed
53.7
“Low flow”
126.3
(11.8)
-
7.6
(1.0)
-
C
Beautifil Flow Plus F03 flowable giom er spatulated
53.7
“Low flow”
103.5
(18.3)
-
6.6
(0.67)
-
D
F03 + 20% low AR (5.2 ) E Glass
43
Paste
95.2
(15.9)
62
11
(1.11)
28.6
E
F03 + 40% low AR (5.2 ) E Glass
32.2
Paste
94.1
(11.5)
80
17
(2.02)
32.6
F
F03 + 60% low AR (5.2 ) E Glass
21.5
“Packable”
81.2
(19.2)
66
20.3
(1.31)
29.4
G
F03 + 5% mid AR (68) S Glass
51
Flowable
181.2
(33.5)
1658
12.1
(0.94)
116.6
H
F03 + 10% mid AR (68) S Glass
48.3
Paste
170.4
(28.2)
772
13.8
(2.39)
78.6
I
F03 + 20% mid AR (68) S Glass
43
Paste
210.7
(48.7)
639.5
13.2
(2.85)
39.6
J
F03 + 10% high AR (640) S Glass
48.3
Paste
247.7
(75.2)
1545
13.3
(1.34)
73.6
Flowable composites are indicated in red in Flowability column. Blue and brown values relate to effective stress yielding and modulus described on lines 364–402.
silane solution consisting of 2% 3-(Trimethyoxysilyl) propyl methacrylate silane (Z-6030 Sigma–Aldrich, MO, USA) in 93% ethanol and 5% deionized water. This solution was titrated to pH 5 with acetic acid. The fibers in solution were stirred for 2 min every 5 min for a total of 44 min before being rinsed several times in deionized water. In order to remove any physisorbed and some of the weakly chemisorbed layers the fibers were then conditioned in 100 ◦ C deionized water for 10 min and then dried at 50 ◦ C for 24 h [20,21].
2.2.
Dental resin composite
The dental composite used in this study Beautifil is classified as a giomer and is composed of BisGMA/TEGDMA resin and pre-reacted glass-ionomer (S-PRG) fluoroaluminosilicate glass filler particles ranging in size from 0.01 to 4.0 (ave. 0.8) m. Manufacturers of these materials claim fluoride release and recharge capabilities while maintaining superior physical properties to poly acid modified resin composites (compomers) [22].
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
1237
Fig. 1 – E glass (a) and S glass (b) as received from manufacturer. Cut S glass fibers (c and d) showing random inter penetrating networks.
This material is manufactured with five viscosities depending on filler ratio ranging from 68.6% volume (83.3% by weight) loaded “sculptable” Beautifil II (group A) to the very low viscosity Beautifil Flowable F10 ( 0.05) than syringed control (group B) suggesting that improvements in flexural properties may be expected with void minimization in the mechanical manufacturing of a reinforced material. In addition it may be expected that fiber surfaces further entrap air so minimizing the volume loading may help optimize properties. Overall, the addition of randomly oriented very short (less than 1 mm) fibers led to significant changes to both strength and modulus of the flowable composite although only the 5% loaded mid aspect ratio group retained flowability. This material had 73% the maximum flexural strength of the best performing test group but with fibers 11% of the length and 50% of the volume. It also outperformed the “sculptable” universal control in strength by 30% (p > 0.05) while containing 62% more resin (51% vs. 31.4%).
6.
Limitations and future direction
The mixing of the mid and high aspect ratio fibers resulted in some clumping and paralleling of fibers as can be seen in Fig. 3c and d. The dispensing of a fiber reinforced resin composite through a nozzle may show a similar result. Further work may look at the influence of fiber aspect ratio, fiber volume fraction and nozzle diameter in delivering a randomly distributed fiber system. In this study, viscosity was assessed
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
visually. Future work will involve quantitative measurement of flowability. This study has shown the significant impact of surface and other imperfections and inclusions on strength properties of dental composites. Therefore, future work may also look at surface manipulation and treatments.
7.
Clinical significance
The aim of any material research is to improve accepted bench top physical properties and practical clinical durability. To our knowledge there are no dental resin composites with flexural strength values above 200 MPa (max. 177 MPa for 82% vol. nano hybrid). Given dentin flexural strength values of 260 MPa being reported it would seem appropriate to mimic this material property as close as possible if stress strain disparity is to be avoided. The addition of fibers have been shown to reliably increase flexural modulus. Given dentin flexural modulus values reported average 17 GPa it may be advantageous to mimic this parameter as well. It may be argued that while fracture toughness is a measure of resistance to the catasrophic propagation of flaws it more a materials strength that influences initiation. Thus fibers may improve damage incidence as well as tolerance.
8.
Conclusions
Within the parameters studied the fiber aspect ratios of 68 and 640 resulted in significant (p < 0.05 and p < 0.001) improvements to the flexural strength of flowable controls even with relatively small volume load. All low aspect ratio (5.2) samples though had a negative effect and fiber volume loading was not significant for strength. As expected with regards to flexural modulus the presence of any volume of stiffer fiber led to a significant increase (p < 0.05) in stiffness even if it weakened the composite (low aspect ratio). Fiber aspect ratio was not significant for modulus.
Acknowledgements Ken Tyler for his time and expertise at the Biomaterials Laboratory, Sydney Dental Hospital. The Australian Center for Microscopy and Microanalysis (University of Sydney), in particular, Drs Ian Kaplin and Patrick Trimby. Shofu (Kyoto, Japan) and Horsley Dental (Australia) for their supply of flowable composite. AGY (South Carolina, USA) for supply of S-2 glass fibers. Owens Corning (Ohio, USA) for E glass fibers.
references
[1] Ilie N, Hickel R. Resin composite restorative materials. Aust Dent J 2011;56(1 Suppl.):59–66. [2] Basaran EG, Ayna E, Vallittu PK, Lassila LVJ. Load bearing capacity of fiber-reinforced and unreinforced composite resin CAD/CAM-fabricated fixed dental prostheses. J Prosthet Dent 2013;109:88–94.
1243
[3] Narva KK, Lassila VL, Vallittu PK. The static strength and modulus of fiber reinforced denture base polymer. Dent Mater 2005;21:421–8. [4] Al-Darwish M, Hurley RK, Drummond JL. Flexure strength evaluation of a laboratory-processed fiber-reinforced composite resin. J Prosthet Dent 2007;97:266–70. [5] Nakamura T, Waki T, Kinuta S, Tanaka H. Strength and elastic modulus of fiber-reinforced composites used for fabricating FPD’s. Int J Prosthodont 2003;16: 549–53. [6] Xu HHK, Schumacher GE, Eichmiller FC, Peterson RC, Antonucci JM, Mueller HJ. Continuous-fiber preform reinforcement of dental resin composite restorations. Dent Mater 2003;19:523–30. [7] Belli S, Cobankara FK, Eraslan O, Eskitascioglu G, Karbhari V. The effect of fiber insertion on fracture resistance of endodontically treated molars with MOD cavity and reattached fractured lingual cusps. J Biomed Mater Res B: Appl Biomater 2006;79B:35–41. [8] Biodental Technologies Internal Data. BDT, Sydney, Australia. [9] Zakaria MR, Najim WA. A comparison of fracture strength of posterior composite and ceramic inlay materials. J Bagh College Dent 2009;21:18–22. [10] Stavridakis MM, Lutz F, Johnston WM, Krejci I. Linear displacement and force induced by polymerization shrinkage of resin-based restorative materials. Am J Dent 2003;16:431–8. [11] Manhart J, Kunzelmann KH, Chen HY, Hickel R. Mechanical properties of new composite restorative material. J Biomed Mater Res Appl Biomater 2000;53:353–61. [12] Ellakwa A, Shortall A, Shehata MK, Marquis PM. The influence of fiber placement and position on the efficiency of reinforcement of fibre reinforced composite bridgework. J Oral Rehabil 2001;28:785–91. [13] Karbhari VM, Strassler H. Effect of fiber architecture on flexural characteristics and fracture of fiber-reinforced dental composites. Dent Mater 2007;23:960–8. [14] Dyer S, Lassila L, Jokinen M, Vallittu P. Effect of cross-sectional design on the modulus of elasticity and toughness of fiber-reinforced composite materials. J Prosthet Dent 2005;94:219–26. [15] Karacaer O, Tezvergil L, Vallittu P. The effect of length and concentration of glass fibers on the mechanical properties of an injection and a compression moulded denture base polymer. J Prosthet Dent 2003;90:385–93. [16] Callaghan DJ, Vaziri A, Nayeb-Hashemi H. Effect of fiber volume fraction and length on the wear characteristics of glass fiber-reinforced dental composites. Dent Mater 2006;22:84–93. [17] Petersen RC. Discontinuous fiber-reinforced composite above critical length. J Dent Res 2005;84:365–70. [18] van Dijken JWV, Sunnegårdh-Grönberg K. Fiber-reinforced packable resin composites in Class II cavities. J Dent 2006;34:763–9. [19] AGY. High Strength Glass Fibers. Technical Paper; 1996. [20] Cheng TH, Jones FR, Wang D. Effect of fiber conditioning on the interfacial shear strength of glass-fiber composites. Compos Sci Technol 1993;48:89–96. [21] Wang D, Jones FR, Denison P. Surface analytical study of the interaction between gamma amino propyl triethoxysilane and E-glass surface. Part I: time-of-flight secondary ion mass spectrometry. J Mater Sci 1992;46:36–48. [22] Itota T, Carrick TE, Yoshiyama M, McCabe JF. Fluoride release and recharge in giomer, compomer and resin composite. Dent Mater 2004;20:789–95. [23] Guth JH. The Mechanism of Asbestos-induced Carcinogenesis: Calcium and Plasma Membrane Integrity.
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Paper presented to the Virginia Academy of Sciences, 59th Annual Meeting, Norfolk, VA, May 14; 1981. Lockey JE, Roggli VL, Hilbert TJ, Rice CH, Levin LS, Borton EK, et al. Biopersistence of refractory ceramic fiber in human lung tissue and a 20-year follow-up of radiographic pleural change in workers. J Occup Environ Med 2012;54:781–8. Chen L, Yu Q, Wang Y, Li H. BisGMA/TEGDMA dental composite containing high aspect-ratio hydroxyapetite nano fibers. Dent Mater 2011;27:1187–95. Alander P, Lassila LVJ, Vallitu P. The span length and cross-section design affect values of strength. Dent Mater 2005;21:347–53. Garoushi S, Vallittu PK, Lassila L. Short glass fiber reinforced restorative composite resin with semi-inter penetrating polymer network matrix. Dent Mater 2007;23:1356–62. Kardos JL. Critical issues in achieving desirable mechanical properties for short fiber composites. Pure Appl Chem 1985;57:1651–7.
[29] Garoushi S, Sailynoja E, Vallittu P, Lassila L. Physical properties and depth of cure of a new short fiber reinforced composite. Dent Mater 2013;29:835–41. [30] Opdam NJM, Roeters JJM, Joosten M, Veeke O. Porosities and voids in class I restorations placed by six operators using a packable or syringable composite. Dent Mater 2002;18:58–63. [31] Cooke TF. Fiber reinforcement high performance. John Wiley and Sons; 1993. p. 217–32. [32] Callister WD. Materials science and engineering: an introduction. John Wiley & Sons; 2007. p. 585–600. [33] Zhu YT, Valdez JA, Shi N, Lovato ML, et al. Influence of reinforcement morphology on the mechanical properties of short fibre composites. In: Processing of Metals and Advanced Materials: Modeling, Design and Properties. The Minerals, Metals & Materials Society; 1998. p. 251–60. [34] Brandt AM. Fibre reinforced cement-based composites after over 40 years of development in building and civil engineering. Compos Struct 2008;86:3–9.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
The effect of fiber aspect ratio and volume loading on the flexural properties of flowable dental composite Paul Shouha, Michael Swain, Ayman Ellakwa ∗ Biomaterials Science, Discipline of Biomaterials, Faculty of Dentistry, The University of Sydney, Sydney Dental Hospital, 2 Chalmers St., Surry Hills, Sydney, NSW 2010, Australia
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To evaluate the efficacy on flexural properties of flowable dental resin composite
Received 25 September 2013
reinforced with short glass fiber of different aspect ratios (ARs) and volume percent load-
Received in revised form
ings. It is hypothesized that with the addition of randomly oriented fibers it is possible to
2 April 2014
significantly improve flexural strength and modulus while maintaining flowability.
Accepted 8 August 2014
Methods. Ten groups of samples with varying glass fiber volume loads (0, 5%, 10%, 20%, 40% and 60%) and three different ARs (5.2, 68 and 640) were tested in three point bending to fracture according to ISO 4049. A flowable resin composite was used as the control and also
Keywords:
as the filled resin composite that was subsequently reinforced with fibers. Load deflection
Flexural
results were used to calculate flexural strength and flexural modulus. SEM images were
Fracture
used to determine the mode(s) of failure, to describe surface features of reinforcement and
Strength
were correlated with force displacement graphs. All results were statistically analysed using
Modulus
ANOVA followed by post hoc Tukey’s test. Level of significance was set at 0.05.
Dental composite
Results. When compared to the “sculptable” control (68.6 vol% filler loaded) results for flex-
Flowable
ural strength varied from a mean reduction of 42% (p > 0.05) for the low AR group to an
Glass fiber
increase of 77% (p < 0.001) for the high AR samples. Flexural modulus results varied from a
Aspect ratio
low of 6.6 [0.67] GPa for the non reinforced spatulated control to 20.3 [1.31] GPa (p < 0.001) for the 60% loaded low AR group. The low fiber loaded mid AR group was still flowable with 49% total loading (5% fiber/44% filler) but gave strength values (181.2 [33.5] MPa) 30% higher than the “sculptable” control (p > 0.05) and comparable modulus. Significance. This study shows that short and very short glass fibers can significantly reinforce flowable dental composite. The fiber’s aspect ratio was shown to be more important than volume loading for flexural strength. It appears possible to produce a light cured short glass fiber reinforced flowable material with superior flexural properties compared to conventional universal composites. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author: Associate professor, Faculty of Dentistry, The University of Sydney, Westmead Oral Health Centre, Level 1, Westmead Hospital, Darcy Road, Sydney, NSW 2145 Australia. Tel.: +61 298457161; fax: +61 296334759. E-mail address: [email protected] (A. Ellakwa) .
http://dx.doi.org/10.1016/j.dental.2014.08.363 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
1.
Introduction
Whether driven by public esthetic demands, concerns over amalgam’s toxicity or trends toward conservative management of dentitions, dental composites have become the material most often employed in the restoration of teeth. These materials are composed of varying amounts of inorganic radio opaque particulate fillers dispersed in a relatively weak organic resin matrix. Bi-functional silane compounds are used to create the interphase between the two. These materials are manipulated directly in the mouth and are set with curing lights of approximately 470 nm wavelength. Their use in stress bearing areas of posterior teeth is becoming an acceptable option but brittleness and a tendency to edge fracture remain important limitations [1]. There has been a long and successful history of high performance fiber reinforcement of industrial composites. This cannot be said of dental composites. Although relatively uncommon clinically, the introduction of fibers have been studied in fixed and removable dental prostheses [2,3]. These fiber systems though are usually continuous and are indirectly manufactured [4–6]. Compared with direct procedures these restoratives require more aggressive tooth preparation. Other limiting factors include significant laboratory fees and the increased probability of handling and processing errors. Chair side reinforcement with continuous fiber composite is still less common. This direct procedure involves combining regular dental composite with sections cut from straight or woven ultra high molecular weight polyethylene fibers. Not only has fiber reinforcement been shown to strengthen and toughen dental composite but direct reinforcing was also found to strengthen restored teeth [7]. Additionally, the fracture type changes from a catastrophic brittle pattern to a more graceful controlled one. Least common are dental composites reinforced with randomly oriented short fibers. These materials claim higher stiffness, fracture toughness and flexural strength. For example: 16 GPa, 3.2 MN/m3/2 and 176 MPa respectively for a 71% volume filled, 20–800 m long E glass rod material [8,9]. They also claim very low polymerization shrinkage [10] but have been found to exhibit poor wear properties due to “plucking” of the relatively large surface exposed fibers and fillers [11]. Variables such as fiber position and orientation have been studied [12–14]. Using chopped glass fibers in denture polymers Karacaer [15] found significant correlation between fiber volume with both flexural modulus and impact strength. Callaghan [16] found that increasing the length of the fibers increased the wear resistance of reinforced specimens. The intent of this study was to evaluate the effect of different fiber aspect ratios and fiber volume loads on the flexural strength and modulus of low viscosity or flowable dental composites. Fiber aspect ratio (AR) is simply the relationship of length to diameter of a fiber given as a number. Some studies have quantified AR as a variable for mechanical properties of dental materials [16–18] but overall there exists very little research in this field and none that we know of with regards to flowable dental materials. The hypothesis
1235
evaluated was that a flowable dental composite may be significantly reinforced by incorporating randomly distributed short and very short glass fibers. Further it was hypothesized that, the amount of fiber required to increase the flexural behavior is small thereby retaining low viscosity and flowability.
2.
Materials and methods
Ten groups (A–J) with ten samples per group were formed. Three control groups A, B and C were not reinforced while groups D–J formed the fiber reinforced samples (Table 1). Group A consisted of a universal high viscosity or “sculptable” dental composite (Beautifil II, Shofu, Kyoto, Japan) while groups B and C consisted of a low viscosity flowable composite (Beautifil F03 Flow Plus). Groups D–F were formed by combining this flowable material (F03 Flow Plus) with low (5.2) aspect ratio fibers of different volume load fractions (20, 40 and 60%). Groups G, H and I combined F03 Flow Plus with mid (68) aspect ratio fibers of three different loads (5, 10 and 20%) while group J utilized high aspect ratio (640) fibers with 10% loading.
2.1.
Fibers
Glass fibers with three different aspect ratios (ARs) were used (Fig. 1). Low AR fibers presented as unsized milled E glass powder from Owens Corning, Ohio, USA. SEM analysis revealed an average length of 89 ± 62 m (range 14–300, n = 400), an average diameter of 17 m and an AR of 5.2 ± 3.65. They were used for three groups (D–F). High AR glass fibers presented as S glass (S2 Glass) chopped strands from AGY, South Carolina, USA and consisted of numerous 10 m diameter parallel filaments cut into short 6.4 mm lengths. These fibers have an AR of 640 without deviation and were used for group J. These S glass strands were also cut (average length 679, range 60–2400, n = 400) to create the third AR of 68 ± 43.5 used in 3 groups (G–I). Both E glass and S glass fibers were used as the availability of very short cut or milled glass fibers is very limited. The prefix E and S stands for electrical grade and structural grade. Their physical properties are as follows: tensile strengths 3.4 GPa vs. 4.9 GPa, moduli 72 GPa vs. 87 GPa and densities of 2.58 g/cc vs. 2.46 g/cc respectively. Both fibers are predominantly aluminosilicate glasses. E glass is high in calcium while S glass is high in magnesium with no boron [19]. Given the similarity in composition, physical properties and surface chemistry it was decided to use milled E glass as a low AR baseline. This material is readily available and is used by manufacturers of “packable” DRCs. The S glass fibers were used uncut in the higher AR sample group. The major difference between the two glasses was thus considered to be diameter and a sufficient spread of aspect ratios was achieved. All fibers were first etched in a 4% potassium dichromate(IV)(K2 Cr2 O7 )/89% sulphuric acid (H2 SO4 ) aqueous solution (Australian Scientific, NSW, Australia) for 4 h then thoroughly washed (until pH returned to 7) in deionized water followed by rinsing in ethanol and drying in an oven at 50 ◦ C for 8 h. Fibers were then immersed in a hydrolyzed
1236
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
Table 1 – Ten groups (n = 10): three controls (A–C); three low aspect ratio (D–F); three mid AR (G–I) and one high AR (J) groups. Mean values and one standard deviation (brackets) are given. Note that only group G remained flowable after fiber incorporation. ef , effective stress yielding; Eef , effective modulus.
Group
Composion
Resin
Flowability
Flexural Strength mean
%
(SD)
MPa
σef
Flexural Modulus mean (SD)
GPa
Eef
A
Beautifil II universal giomer composite
31.4
“Sculptable”
139.8
(10.6)
-
12.2
(0.74)
-
B
Beautifil Flow Plus F03 flowable giom er syringed
53.7
“Low flow”
126.3
(11.8)
-
7.6
(1.0)
-
C
Beautifil Flow Plus F03 flowable giom er spatulated
53.7
“Low flow”
103.5
(18.3)
-
6.6
(0.67)
-
D
F03 + 20% low AR (5.2 ) E Glass
43
Paste
95.2
(15.9)
62
11
(1.11)
28.6
E
F03 + 40% low AR (5.2 ) E Glass
32.2
Paste
94.1
(11.5)
80
17
(2.02)
32.6
F
F03 + 60% low AR (5.2 ) E Glass
21.5
“Packable”
81.2
(19.2)
66
20.3
(1.31)
29.4
G
F03 + 5% mid AR (68) S Glass
51
Flowable
181.2
(33.5)
1658
12.1
(0.94)
116.6
H
F03 + 10% mid AR (68) S Glass
48.3
Paste
170.4
(28.2)
772
13.8
(2.39)
78.6
I
F03 + 20% mid AR (68) S Glass
43
Paste
210.7
(48.7)
639.5
13.2
(2.85)
39.6
J
F03 + 10% high AR (640) S Glass
48.3
Paste
247.7
(75.2)
1545
13.3
(1.34)
73.6
Flowable composites are indicated in red in Flowability column. Blue and brown values relate to effective stress yielding and modulus described on lines 364–402.
silane solution consisting of 2% 3-(Trimethyoxysilyl) propyl methacrylate silane (Z-6030 Sigma–Aldrich, MO, USA) in 93% ethanol and 5% deionized water. This solution was titrated to pH 5 with acetic acid. The fibers in solution were stirred for 2 min every 5 min for a total of 44 min before being rinsed several times in deionized water. In order to remove any physisorbed and some of the weakly chemisorbed layers the fibers were then conditioned in 100 ◦ C deionized water for 10 min and then dried at 50 ◦ C for 24 h [20,21].
2.2.
Dental resin composite
The dental composite used in this study Beautifil is classified as a giomer and is composed of BisGMA/TEGDMA resin and pre-reacted glass-ionomer (S-PRG) fluoroaluminosilicate glass filler particles ranging in size from 0.01 to 4.0 (ave. 0.8) m. Manufacturers of these materials claim fluoride release and recharge capabilities while maintaining superior physical properties to poly acid modified resin composites (compomers) [22].
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
1237
Fig. 1 – E glass (a) and S glass (b) as received from manufacturer. Cut S glass fibers (c and d) showing random inter penetrating networks.
This material is manufactured with five viscosities depending on filler ratio ranging from 68.6% volume (83.3% by weight) loaded “sculptable” Beautifil II (group A) to the very low viscosity Beautifil Flowable F10 ( 0.05) than syringed control (group B) suggesting that improvements in flexural properties may be expected with void minimization in the mechanical manufacturing of a reinforced material. In addition it may be expected that fiber surfaces further entrap air so minimizing the volume loading may help optimize properties. Overall, the addition of randomly oriented very short (less than 1 mm) fibers led to significant changes to both strength and modulus of the flowable composite although only the 5% loaded mid aspect ratio group retained flowability. This material had 73% the maximum flexural strength of the best performing test group but with fibers 11% of the length and 50% of the volume. It also outperformed the “sculptable” universal control in strength by 30% (p > 0.05) while containing 62% more resin (51% vs. 31.4%).
6.
Limitations and future direction
The mixing of the mid and high aspect ratio fibers resulted in some clumping and paralleling of fibers as can be seen in Fig. 3c and d. The dispensing of a fiber reinforced resin composite through a nozzle may show a similar result. Further work may look at the influence of fiber aspect ratio, fiber volume fraction and nozzle diameter in delivering a randomly distributed fiber system. In this study, viscosity was assessed
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 1234–1244
visually. Future work will involve quantitative measurement of flowability. This study has shown the significant impact of surface and other imperfections and inclusions on strength properties of dental composites. Therefore, future work may also look at surface manipulation and treatments.
7.
Clinical significance
The aim of any material research is to improve accepted bench top physical properties and practical clinical durability. To our knowledge there are no dental resin composites with flexural strength values above 200 MPa (max. 177 MPa for 82% vol. nano hybrid). Given dentin flexural strength values of 260 MPa being reported it would seem appropriate to mimic this material property as close as possible if stress strain disparity is to be avoided. The addition of fibers have been shown to reliably increase flexural modulus. Given dentin flexural modulus values reported average 17 GPa it may be advantageous to mimic this parameter as well. It may be argued that while fracture toughness is a measure of resistance to the catasrophic propagation of flaws it more a materials strength that influences initiation. Thus fibers may improve damage incidence as well as tolerance.
8.
Conclusions
Within the parameters studied the fiber aspect ratios of 68 and 640 resulted in significant (p < 0.05 and p < 0.001) improvements to the flexural strength of flowable controls even with relatively small volume load. All low aspect ratio (5.2) samples though had a negative effect and fiber volume loading was not significant for strength. As expected with regards to flexural modulus the presence of any volume of stiffer fiber led to a significant increase (p < 0.05) in stiffness even if it weakened the composite (low aspect ratio). Fiber aspect ratio was not significant for modulus.
Acknowledgements Ken Tyler for his time and expertise at the Biomaterials Laboratory, Sydney Dental Hospital. The Australian Center for Microscopy and Microanalysis (University of Sydney), in particular, Drs Ian Kaplin and Patrick Trimby. Shofu (Kyoto, Japan) and Horsley Dental (Australia) for their supply of flowable composite. AGY (South Carolina, USA) for supply of S-2 glass fibers. Owens Corning (Ohio, USA) for E glass fibers.
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